Xianrong Liu

Double Ionization of Nitrogen from Multiple Orbitals

In intense femtosecond laser fields, molecules could be tunnel ionized from multiple valence orbitals, which produces molecular ions in different electronic states. In this article, we have used a reaction microscope to study double ionization of nitrogen by intense femtosecond laser pulses. It is found that some doubly charged molecular ions N(2)(2+) are metastable while others dissociate through charge symmetric dissociation (N(2)(2+) --> N(+) + N(+)) or charge asymmetric dissociation (N(2)(2+) --> N(2+) + N). The kinetic energy releases and angular distributions of atomic ions are measured for the dissociation channels. With the aid of the CASSCF and MRCI calculations, the electronic states are identified and the contributions of the valence orbitals are discussed for these dissociated molecular dications.

Coincidence imaging of photoelectrons and photo-ions of molecules in strong laser fields

In recent years, reaction microscopes have become a powerful technique to image ultrafast dynamics in atoms and molecules. Here, we present the specific details of our reaction microscope that was designed to study molecular dynamics driven by intense femtosecond laser pulses. A supersonic molecular beam and laser focusing system was specially designed to confine molecules to the peak intensity of the laser focus. Reaction channels can be precisely identified and three-dimensional momentum vectors can be accurately acquired for correlated reaction products with high resolution. The molecular structure information can be extracted based on the experimentally measured momentum vectors of the photoelectrons and the photo-ions that are generated in the laser–molecule interaction.

Fully differential measurement on above-threshold ionization of CO and CO 2 molecules in strong laser fields

We present a fully differential measurement on above-threshold ionization of diatomic (CO) and linear triatomic (CO2) molecules in strong femtosecond laser fields (25fs, 795nm) at a laser intensity of ∼8×1013W/cm2. The photoelectron longitudinal momentum distribution of CO in the laser polarization direction shows a broader distribution near zero momentum in the laser polarization plane with a shoulder around 0.38a.u., whereas it shows a clear maximum for CO2. The relative photoelectron yields near zero longitudinal momentum are less dependent on the laser intensity. The difference may result from the molecular structure. Even though molecular targets have different binding energy and outer orbitals, the near-threshold photoelectrons have similar angular distribution, and the dominant angular momentum of the near-threshold photoelectron is L=4 (ng Rydberg state) for both targets.

Multiple ionization of oxygen studied by coincident measurement

We experimentally study double and triple ionization of oxygen using a reaction microscope. The kinetic energy releases (KERs) and angular distributions are obtained through coincidentally measuring the ionic fragments of doubly or triply charged parent ions. The pathway O(2+)2 ? O (+) O(+)?proceeds through some excited electronic states. The KERs exhibit definite structures independent of the laser intensity and the pulse duration. However, the angular distribution of coincident O(+) reflects the symmetry of the highest occupied molecular orbital (HOMO) only for few-cycle laser pulses at low intensity. The pathways O(2+) 2 --> O(2+) +O and O(3+) 2 --> O(2+) + O(+) occur through some repulsive states. The KERs show a single broad peak and decrease with increasing the pulse duration. The decrease of KER comes from the stretch of the internuclear distance in intense laser fields.

Steering of Molecular Multiple Dissociative Ionization with Strong Few-Cycle Laser Fields

Coherent control is an implicit goal of quantum physics and quantum chemistry, which have been significantly developed in utilizing light to control over the dynamics of atomic and molecular systems. We show that the process of multiple dissociative ionization of carbon monoxide (CO) molecules is controllable using an intense phase-stabilized few-cycle laser field (4.2 fs, 740 nm, \(6 \times 1{0}^{14}\,\mathrm{W}/{\mathrm{cm}}^{2})\). We demonstrate that the controllable emission direction of \({\mathrm{C}}^{2+}\) from charge asymmetrical dissociation and ionization of CO dications is out of phase in a linearly polarized laser field. The strong coupling between the dissociative channels is explained with the mechanisms of recollision excitation and recollision ionization. The competition between excitation and ionization in a recollision process leads to the opposite asymmetrical property. The results provide an insight into the controllable attosecond dynamics of multiple dissociative ionization of a complex molecule.